The nuclear envelope separating the major compartments of the eukaryotic cell breaks down at the beginning of mitosis and is reformed around the separated chromosomes at the end of mitosis. Breakdown and reconstruction must be regulated precisely. This proposal aims at elucidating: functions and structural changes that are dependent on and independent of a reconstructed nuclear envelope; the signals or factors that remain on the mitotic nuclear membranes; and possibly those proteins that are reassociating with the nuclear membranes during reconstruction. Technically, monoclonal antibodies against the membrane-chromatin connecting antigens will be microinjected and the cells observed ultrastructurally. Mitotic nuclear membranes will be obtained by affinity chromatography to study events of nuclear envelope reconstruction. These cell biological investigations will establish the framework for the future search for mitosis-inducing factors and their antagonists.